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The novel source of relativistic electron beams REGAE is a joint project of the CFEL partners Max Planck Society, University of Hamburg and DESY. The nearly ten-metres-long facility generates ultrashort electron pulses of extremely high quality for time-resolved diffraction experiments on crystallised material samples, liquids and surfaces.

Just as scientists direct light from storage rings or free-electron lasers onto material samples to determine their molecular structure by means of the generated diffraction patterns, such experiments are also possible with electrons. Because of the small penetration depth of electrons compared to X-rays, they are particularly suitable for nanomaterials. REGAE thus aims at both time-resolved structural studies of crystallized material samples and in-situ studies of liquids, surfaces and liquid phases in nanochemistry.

Molecular movies with electrons

To enable the motion of atoms to be tracked with high time resolution, REGAE is set to produce electron pulses with a duration of only about 10 femtoseconds. The diameter of an electron bunch will be half a millimetre; the length only one fifth of the diameter of a human hair.

The experimental method used at REGAE is comparable to that of a transmission electron microscope. When the electrons hit the object under investigation, they are diffracted by the molecular structure of the sample. The diffraction angles – which are a measure of the interatomic distances in the sample – are determined with high precision using an innovative CCD detector. Their evaluation reveals an image of the atomic structure of the sample at the time of its interaction with the electron bunch. As the electron pulses last only femtoseconds, the researchers can use them to directly observe the motion of the atoms in the molecules on this time scale.

Test facility for accelerator development

REGAE comprises a laser-driven electron source similar to the one used in the FLASH free-electron laser. The electron bunches are accelerated to an energy of 5 mega-electronvolts (MeV) and packed tightly together by means of a special buncher cavity, which accelerates the particles in rear part of the bunch while slowing down those in the front. The optical laser that triggers the electron bunch in the source can simultaneously be used to excite the sample. This allows so-called pump-probe experiments, in which the laser pulse excites specific processes in the sample that are then probed with a subsequent electron pulse.

Because of the high demands placed on the quality of the electron beam, REGAE serves not only as a research facility for electron diffraction experiments but also as a test facility for accelerator development. As a further highly interesting future perspective, it is planned to inject the electron beam from REGAE into a plasma wave excited by a high-power laser in order to test acceleration at ultrahigh gradients.